Necrotizing enterocolitis (NEC) is a life-threatening, gastrointestinal emergency that occurs in 7 to 10% very low birth weight (VLBW, <1500 g) infants. Despite improvements in neonatal intensive care, the birth weight-specific incidence of NEC has not changed over the past 2 decades. However, the total burden of disease is increasing due to increased survival of very immature infants. Mortality remains high and survivors experience significant morbidity including post-surgical short bowel syndrome, poor growth, prolonged hospitalization, and long-term neurodevelopmental impairments. Prematurity, gut dysbiosis, and impaired intestinal barrier function are three key factors in NEC pathogenesis. The co-existence of these three factors leads to bacterial translocation across an impaired intestinal mucosal barrier, intense local and systemic inflammatory responses, and intestinal necrosis typical of NEC. To mitigate the risk of NEC, the use of probiotics has been proposed to correct the intestinal barrier dysfunction and improve gut dysbiosis. Although there was a 30% reduction in NEC incidence in multiple randomized clinical trials of probiotics to prevent NEC, various formulations, doses, and duration of therapy were used, infants <1000 g BW with the highest NEC incidence were underrepresented, and no FDA-approved products are available to assure quality and safety under good manufacturing practices. There is an urgent need for well-designed trials with proper regulatory oversight to address the dose of a specified probiotic product, in a high-risk, well characterized, population. Our long-term goal is to determine the safety and efficacy of Lactobacillus rhamnosus GG (LGG) to prevent NEC in VLBW infants. The overall objective of the current proposal is to conduct an open label, safety and pilot dose escalation study of LGG to collect information needed to design Phase III safety and efficacy trials of LGG in the preterm population. Specifically, the proposed study will assess the utility of an easily collected novel biomarker zonulin to monitor intestinal permeability, and provide information on the safety and acceptability of administering 3 different doses of LGG to VLBW infants, the optimal dose of LGG to improve intestinal barrier function, and the impact of LGG on GI microbiota. The specific aims will address the central hypotheses that 1) increased levels of biomarkers (serum zonulin, Lactulose/Mannitol urine ratio, and/or fecal a-1 anti-trypsin) will identify infants with increased intestinal permeability; and 2) oral administration of LGG in VLBW infants with elevated intestinal permeability biomarkers will improve gut barrier function and favor the establishment of a beneficial gut microbiota. Development of biomarkers to identify infants with increased intestinal permeability who are at high risk for NEC will facilitate clinical trials of potential preventive therapies. Probiotic therapy is a promising, low-cost, and likely safe intervention to reduce intestinal permeability in high-risk infants. The data generated from the proposed studies will directly lead to decisions concerning future randomized safety and efficacy trials of LGG to prevent NEC in the preterm population.